4th International Conference on
Earthquake Geotechnical Engineering
June 25-28, 2007
Paper No. 1494
THE SEISMICITY OF GAZİANTEP, TURKEY
Ali Fırat ÇABALAR 1
ABSTRACT
Although some differences exist between the regional kinematic models in the literature, many studies
have proposed that the boundaries between African, Arabian and Anatolian Plates meet at a triple
junction in southern-central Turkey. Following the overview made of the previous works on the
tectonic models, historical seismicity and the known slip rates along the faults in the region, the study
has aimed to point out the northern part of the Dead Sea Fault Zone (DSFZ) in Turkey and Syria, and
the western part of the East Anatolian Fault in south-eastern Turkey. It has been seen that the leftlateral slip on the DSFZ is associated with earthquakes of magnitude ~7.5 occurring at intervals of
several hundred years on each fault segment. Also, it is interpreted that a possible earthquake could
have significant economic and social effects on cities located in the region. The city of Gaziantep, with
a population of over 1.000.000, is one of the cities in this region and located at approximately 37.08
N- 37.38 E. The study investigates the seismicity and the earthquake hazard of Gaziantep. The
seismicity of Gaziantep has been investigated by using the earthquakes M ≥ 3 that occurred in the
region for the time interval 1973-2003. A fairly simple study has also been investigated using timeindependent probabilistic model to predict the future earthquakes activities. Evidently, the paper is
intended to serve as an additional reference for more advanced approaches and to stimulate
discussions and suggestions on region, particularly on the city of Gaziantep.
Keywords: Gaziantep, seismicity.
INTRODUCTION
Turkey lies among the Mediterranean part of the Alpine-Himalayan orogenic system. The Alpine
orogeny is result of the compressional motion between Africa and Europe; however the Himalayan
orogeny is produced as a result of the India-Asia collision. The seismicity distribution among the
Alpine-Himalayan system is not homogenous, which concentrates mostly along the plate margins. The
African, Arabian and Eurasian plates are involved in the tectonics of the Mediterranean region
(McKenzie, 1970); however the eastern Mediterranean is more complicated. The reason of the local
increase in seismic activity in the region may be because of the rapidly moving smaller plates (Erdik et
al., 1985). The reader is referred to the plate tectonics models for this region for details (i.e.,
McKenzie, 1972, Alptekin, 1973, Dewey and Şengör, 1979, Westaway, 2003).
The Gaziantep Basin is situated in southern Turkey, to the south of the suture zone that formed during
the collisions of the Arabian and Anatolian plates in late Cretaceous (Maastrichtian) and Miocene
times (Coşkun and Coşkun, 2000) (Figure 1). The City of Gaziantep is located between the lands of
the Mediterranean and Mesopotamia and had been often chosen to be the settlement and transition
place of mankind. Today, its population (over 1.000.000), industrial importance and tourism potential
make the city metropolitan. Gaziantep City is the most developed city of the GAP (South Eastern
Anatolian Project) region in industry and commerce as an export gate by its tens of hundreds different
products exported to more than 100 countries. The project called Southeastern Anatolian Project
1
Research Assistant, Geotechnical Engineering Division, Department of Civil Engineering, University
of Gaziantep, Turkey, Email: a_cabalar@yahoo.com
(GAP: Turkish acronym) is a multi-sectoral and regional development project including the
construction of 19 hydraulic power plants, 22 dams, 26.5 km long irrigation tunnels 25 ft in diameter
on an area extending 74,000 km2, one tenth of the country (Çetin et al. 2000).
Figure 1 Location map showing the different plates that influenced the structural evolution of
southeast Turkey (Coşkun and Coşkun, 2000).
SEISMICITY OF THE SOUTHERN-CENTRAL TURKEY AND WESTERN SYRIA
The northern DSF and western EAF regions are located between the Mediterranean and Mesopotamia
and had been often chosen to be the settlement and transition place of mankind. Although earlier
DSFZ seismicity less well documented, with some discrepancies between different studies, historical
evidence for earthquake is fairly abundant. Ambraseys and Barazangi (1989), for example, reported
destructive earthquakes in 1202 and 1759 in Bekaa Valley (33 oN- 34.5 oN), in 1157, 1170, 1404,
1407, 1796, and 1872 in Gharb Fault (34.5 oN- 37 oN), in 1822 in Karasu Fault Zone (36.5 oN- 37 oN).
Also, documented seismicity since 1500 between Karliova and Kahramanmaras has had approximately
Mo=200x1018 Nm, giving a spatially averaged left-lateral slip rate of approximately 2.2 mm yr-1
(Westaway, 1994). Ambraseys (1989) and Taymaz et al., (1991) reported major historical earthquakes
in 1544, 1789, 1874, 1875, 1879. The earthquakes of 1905 and 1972 were the strongest EAFZ events
of the 20th century. In addition, and earthquake having 6.6 magnitude in 1975 near the northern part of
the EAFZ and 6.4 magnitude in 2003 in Bingol Province.
Seismicity in western part of the EAFZ and the northern DSFZ were described by many authors
emphasized that seismic activations in both zones may be interrelated (e.g., Poirier et al., 1980, BenMenahem, 1979, 1981, 1991). Before 20th century, destructive earthquakes repeatedly occurred in the
EAF and DSF zones. The last activation in both zones took place in the 19th century, with earthquake
having 7 or more magnitudes in the EAFZ in 1874 and 1983, in the DSFZ in 1822 and 1872. The
association of the largest historical earthquakes (i.e., Mw≥7 and 6<Mw<7) in the western EAFZ and
north part of the DSFZ is given in Figure 2, which is based on the investigation of various studies by
Balakani and Moskvina (2004). The reader is referred to the original study by Balakani and Moskvina
2
(2004) for further details on the earthquakes records. Map of significant fault segments of the region is
also presented in Figure 3 to have a greater understanding on some of the locations.
Turkey
39
1789
1893 1905
1544
1513
995
1874
1114
1114
37
1866
1971
1408
1822
1872
1408
115
1170
Syria
1157
35
Mediterranean Sea
Mw≥7
6<Mw<7
1202
Jordan
1759
Iraq
Lebanon
33
35
37
39
41
Figure 2 The long-term seismicity of the region (modified from Balakani and Moskvina, 2004).
METHOD AND ANALYSIS
Probabilistic Seismic Hazard Analysis (PSHA) provides a framework in which size, location, and rate
of recurrence of earthquakes can be identified, quantified, and combined in a rational way to have a
more understanding of the seismic hazard. The steps taken to perform PSHA are (i) to use geological
data and the historical earthquake record to define the locations of earthquake sources and likely
magnitudes and frequencies of earthquakes that may be produced by each source, (ii) to estimate the
ground motions that cover the entire region.
Estimation of future seismic activity is based on the rates of past earthquakes as determined from
earthquake catalogs. Even small earthquakes that are not felt although detected by seismographs may
help to make a more realistic judgment on these rates form the frequency-magnitude distributions of
past seismicity. The investigation presented here is based on the readily available reference that is
cited. The catalog compiled from the USGS covers and area from 36.5oE to 38.5oE longitude and
36.5oN to 38.5oN latitude, and is complete for earthquakes of magnitude 3 and greater from 1973 to
2003. Using data of a larger time period will be used in further research regarding this subject. A basic
assumption of the author’s seismic hazard methodology is that earthquake sources are independent.
Therefore, a catalog that is used to estimate future seismic activity must be free of dependent events
such as aftershocks and foreshocks.
3
Karasu Fault Zone
Gharb Fault
Bekaa Valley
Figure 3 Map of significant fault segments of the region (modified from Westaway, 2003).
Magnitude-Frequency Relations
Gutenberg and Richter (1956) developed an empirical formula relating the magnitude M with
corresponding frequency N. The number of exceedances of each magnitude is divided by the length of
the time period to define a mean annual rate of exceedances of an earthquake. The annual rate of
exceedances for a particular magnitude is referred as the return period of earthquakes exceeding that
magnitude. A plot of the logarithm of the annual exceedances rate against earthquake magnitude gives
a linear relationship. The Gutenberg and Richter formula is as follow;
logN = a – bM
(1)
The constant a depends on the period of observation, on the size of the investigated area and on the
level of seismic activity. The constant b depends on the tectonic properties on the investigated area.
The parameters of the magnitude-frequency relation are calculated using the least square method.
Normal and cumulative frequency values for the investigated region have been determined with the
0.25 magnitude increment (Table 1).
4
Table1 Normal and cumulative frequency values and logN with 0.25 increment in magnitude.
ΔM
3.0 - 3.25
3.25 - 3.5
3.5 - 3.75
3.75 - 4.0
4.0 - 4.25
4.25 - 4.5
4.5 - 4.75
4.75 - 5.0
5.0 - 5.25
5.25 - 5.5
5.5 - 5.75
5.75 - 6.0
Normal frequency
1
5
6
8
13
15
13
1
3
0
0
2
logN
0
0.70
0.78
0.90
1.11
1.18
1.11
0
0.48
0
0
0.30
Cumulative frequency
67
66
61
55
47
34
19
6
5
2
2
2
logN
1.83
1.82
1.79
1.74
1.67
1.53
1.28
0.78
0.70
0.30
0.30
0.30
The author has used logN values of cumulative frequency in Table 1 for calculating a and b constants.
The results given in Table 1 covers the results plotted in the Figure 4, also gives some other data out of
the USGS catalogue (i.e., normal frequency). Therefore, it seems to be useful to present the Table 1
here in this study so that could make the processing of basic data wider to the reader.
As can be seen from the Figure 4, the magnitude-frequency relation for the investigated area is as
follow;
logN = 4.32 – 0.7M
(2)
The line seen on Figure 4 has been plotted based on an average value in an every interval shown in
Table 1, which is 0.25 in magnitude. Therefore, note that the first value at the x-axis is 3.125 instead
of 3.0, which is the corresponding value between 3.0 and 3.25.
Earthquake Hazard
It is necessary to know the probability of occurrence of earthquake in a given time interval. This
probability is called as earthquake hazard. The recurrence of earthquakes is most widely described by
a Poisson model. The Poisson model consists of independent events that occur randomly in time,
where the probability of an event occurring in an interval dt is vdt and v is the average number of
events that occur per second. The probability of exactly k events occurring during a given time interval
of length T is
P (k ) =
(vT ) k − vT
e
k!
(3)
The average number of events occurring during the time interval T is vT while the standard deviation
is vT .
Poisson model possess the following properties (Kramer, 1996);
(i) The number of occurrences in a time interval are independent of the number that occur in any other
time interval.
(ii) The probability of occurrence during a short time interval is proportional to the length of the time
interval.
(iii) The probability of more than one occurrence during a short time interval is negligible.
5
2.5
log N (cumulative frequency)
2
1.5
logN = 4.32-0.7M
1
0.5
0
2.5
3
3.5
4
4.5
5
5.5
6
Magnitude (M)
Figure 4 Application of Gutenberg-Richter law to the seismicity data on the studied region.
The probabilities of earthquake occurrence for the investigated area are calculated for periods of T= 5,
10, 15, 20, 25, 30, and 40 years, and magnitudes of M= 3.0, 3.25, 3.5, 3.75, 4.0, 4.25, 4.5, 4.75, 5.0,
5.25, 5.5, 5.75, and 6.0 are presented in Table 2. The mean return periods of earthquakes have been
also calculated. From the table, such as; the probability of an earthquake occurrence of equal or greater
than magnitude 5.5 in 10 years can be found as 45 percent and, 16 years return period.
It is observed that there is an agreement between the results given by the author in Table 2 and the
results obtained by following the method described in Alptekin (1978) that could make the details of
the procedure pursued wider to the reader.
Table 2 The computed earthquake risk for the investigated area.
M
(magnitude)
5
(year)
10
(year)
15
(year)
20
(year)
25
(year)
30
(year)
40
(year)
3
3.25
3.5
3.75
4
4.25
4.5
4.75
5
5.25
5.5
5.75
6
1
0.99
0.99
0.99
0.96
0.89
0.78
0.64
0.49
0.36
0.26
0.18
0.12
1
1
1
0.99
0.99
0.98
0.95
0.87
0.74
0.59
0.45
0.33
0.23
1
1
1
1
0.99
0.99
0.98
0.95
0.87
0.74
0.59
0.45
0.33
1
1
1
1
0.99
0.99
0.99
0.98
0.93
0.83
0.70
0.55
0.42
1
1
1
1
1
0.99
0.99
0.99
0.96
0.89
0.78
0.63
0.49
1
1
1
1
1
0.99
0.99
0.99
0.98
0.93
0.83
0.70
0.55
1
1
1
1
1
1
0.99
0.99
0.99
0.97
0.91
0.80
0.66
6
Return
period
(year)
0.29
0.44
0.65
0.97
1.46
2
3
4
7
10
16
24
36
CONCLUSIONS
Following the disastrous İzmit and Düzce earthquakes on the North Anatolian Fault (NAF) in 1999,
the earthquake hazard in Turkey, particularly İstanbul in northwest of Turkey, has become a great
concern. However, this study has aimed to present an investigation on seismicity on the northern part
of the Dead Sea Fault Zone in southern Turkey. Investigating on the region has been concluded that a
seismic hazard analysis in detail is needed for possible earthquakes on Northern Dead Sea Fault Zone.
Although there are some differences between the kinematic models as compared by Yurtmen et al.
(2002), Coşkun and Coşkun (2000) had concluded that the faulting influencing the structural evolution
of Gaziantep Basin is the Dead Sea Fault Zone. As a preliminary study, the parameters of magnitudefrequency relation and an earthquake risk were determined for the city of Gaziantep located in the
Gaziantep Basin.
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8